1. Field of the Invention
The present invention relates to an electric double-layer capacitor and a method for producing the electric double-layer capacitor, and more particularly, to a lithium ion capacitor, and a method for producing the lithium ion capacitor.
2. Description of the Related Art
A general electric double-layer capacitor is an electric storage device including a sheet-like positive electrode and a sheet-like negative electrode each having an electrode layer containing a carbon material such as activated carbon and binders, a porous separator which separates both of these electrodes facing each other and electrically isolates these electrodes from each other, and an electrolyte impregnating the members. The electric storage device uses the electrostatic capacity of the electric double-layer, which is generated at the interface between the positive electrode or the negative electrode and the electrolyte. The electric double-layer capacitor has advantages in that the electric double-layer capacitor is capable of inputting and outputting a large current because no chemical reaction occurs at the time of charging and discharging, and that the lifetime is long because deterioration caused by charging and discharging is small.
Further, an energy quantity E which is stored in the electric double-layer capacitor has a relationship of E=½ CV2 between electrostatic capacity C and applied voltage V. For that reason, particularly, withstand voltage is improved to increase usable voltage, thereby making it possible to improve energy density.
In recent years, as a technology for dramatically improving the withstand voltage of electric double-layer capacitors, attention has been paid to lithium ion capacitors in which the carbon material (negative electrode layer) of a negative electrode is doped with lithium (lithium is occluded or inserted into the carbon material) to decrease the negative electrode potential, thereby making it possible to apply a higher voltage. Lithium ion capacitors are different in configuration from normal electric double-layer capacitors in that negative electrodes containing the carbon material which may be doped with lithium and a metal current collector such as copper which is difficult to alloy with lithium, and the electrolyte containing lithium ions therein are used.
One type of lithium ion capacitor is configured in such a manner that a metal lithium sheet as a lithium ion supply source is disposed on the exterior of an electrode laminate and is brought into contact with the negative electrode electrochemically, and the negative electrode is gradually doped with lithium that has been eluted from the metal lithium sheet (for example, refer to JP 2006-286919 A). In this lithium ion capacitor, the metal lithium sheet and the negative electrode are electrically connected to each other. For that reason, the elution of lithium ions and doping of the negative electrode (specifically, negative electrode layer) with the lithium ions are advanced with a potential difference between the metal lithium sheet and the negative electrode as a drive force. In this situation, because an ion conduction path is required to diffuse the eluted lithium ions over the entire electrode laminate, the entire electrode laminate must be made of a porous material. For that reason, a metal sheet that has been made porous in advance such as an expanded metal or a punched metal needs to be used as the electrode current collector instead of the nonporous metal sheet which is used in normal battery electrodes.
However, in the lithium ion capacitor disclosed in JP 2006-286919 A, the diffusion distance between the metal lithium which is a lithium ion supply source and the negative electrode layer which is in the vicinity of the center of the electrode laminate is long. This leads to the problem that it takes a long period of time to dope the negative electrode layer with lithium ions, and another problem that the negative electrode layer cannot be evenly doped with the lithium ions. These problems are caused by fact that the lithium ion supply source is disposed on an exterior of the electrode laminate, and holes in the electrode current collector (porous metal sheet) are completely blocked by the electrode layer, in the lithium ion capacitor of JP 2006-286919 A.